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Gravitational attraction of two objects

  1. Nov 27, 2006 #1
    okay so this is my first time posting anything here so forgive me if i've done something wrong (and do let me know). but i've been struggling with this problem now for a couple days and it seems like the answer is on the tip of my tongue but i can't come up with the steps. so if anyone could give me a clue, that would be great. here goes:
    Two spherical objects have a combined mass of 150kg. The gravitational attraction between them is 8.00X10^-6 N when their centers are 20 cm apart. What is the mass of each?
    I'm sure i'm missing something really simple so any help is appreciated.
    thanks.
     
  2. jcsd
  3. Nov 27, 2006 #2

    marcusl

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    Try writing down Newton's law of universal gravitation. That gives you a relation between m1 and m2. The fact that they sum to 150kg gives you a second relation. You should be able to solve it from there.
     
  4. Nov 27, 2006 #3
    the part i don't get is the relation between the sums. i know
    (G*M1*M2)/r^2=F of m1 on m2,
    and i've been thinking about this for a while now and it's driving me crazy. i can't find a relation between the sum and the product.
     
  5. Nov 27, 2006 #4

    marcusl

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    Put m1 = 150 - m2 into your equation above and solve the quadratic for m2.
     
  6. Nov 27, 2006 #5
    If quadratic is too anoying you could do algebra to isolate the varible, thats what ive always done.
     
  7. Nov 28, 2006 #6
    awesome, i feel really dumb now:rolleyes:. thanks a bunch though.
     
  8. Nov 28, 2006 #7

    marcusl

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    I don't think you can get around solving a quadratic..
     
  9. Nov 28, 2006 #8
    use a calculator. obviously technology is no substitution for knowing how to solve it with pencil and paper. but a calculator will do it, and fast mind you.
     
  10. Nov 28, 2006 #9

    I just looked at it quick, you may be right. I've just never run across a problem in my physics class in which you cant isolate one variable.
     
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